Phosphaethynolate

[7] Similar studies were performed on derivatives of this structure and the results indicated that dimerisation to form a four-membered Li ring is favoured by this molecule.

[5] Ten years later, in 2002, Westerhausen et al. published the use of Becker's method to make a family of alkaline earth metal salts of PCO (see Scheme 2); this work involved the synthesis of the magnesium, calcium, strontium and barium bis-phosphaethynolates.

[5][8] Like the salts previously reported by Becker, the alkali-earth metal analogues were unstable to moisture and air and thus were required to be stored at low temperatures (around −20 °C) in dimethoxyethane solutions.

[9] The authors noted that this sodium salt could be handled in air as well as water without major decomposition; this emphasises the significance of the accompanying counter cation in stabilisation of PCO.

[6][9] Direct carbonylation was a method also employed by Goicoechea in 2013 in order to synthesis a phosphaethynolate anion stabilised by a potassium cation sequestered in 18-crown-6 (see Scheme 4).

[11] The two dominant resonance forms of the phosphaethynolate anion localise negative charge on either the phosphorus or oxygen atoms meaning both are sites of nucleophilicity.

[12] The group's studies used a Re(I) complex and the analysis of its bonding parameters and electronic structure showed that the phosphaethynolate anion coordinated in a bent fashion.

[3] Initially when reacting the anion with triorganyl silicon compounds, it binds via the oxygen forming the kinetic oxyphosphaalkyne product.

[15][16] In 2014, Grutzmacher et al. reported that an imidazolium salt would react with the phosphaethynolate anion to produce a phosphinidine carbene adduct.

[15] The results of these investigations suggested that the lowest energy and therefore most likely pathway involves PCO acting as a Brønsted base initially deprotonating the acidic imidazolium cation to generate the intermediate phosphaketene, HPCO.

[1][15][17] The highly unstable protonated PCO remains hydrogen bonded to the newly produced N-heterocylic carbene prior to rearrangement and formation of the observed product.

[10] They found that the anion could react in a [2+2] fashion with a diphenyl ketene to produce the first isolatable example of a four-membered monoanionic phosphorus containing heterocycle.

[10] Cycloaddition reactions involving the phosphaethynolate anion have also been shown by Grutzmacher and co-workers to be a viable synthetic route to other heterocycles.

[1][18] A large part of the research involving PCO is now looking into utilising the anion as a synthetic building block to derive phosphorus containing analogues of small molecules.

[19] Generating acylphosphines in this manner is considered a much milder route than other current strategies that require multi-step syntheses involving toxic, volatile and pyrophoric reagents.

[7] In addition, NBO analysis highlights that the greatest electron delocalisation within the anions stems from the donation of an oxygen lone pair into the E−C π antibonding orbital.

[11] This is the result of a reduced difference in electronegativity between E and X thus neither atom offers a substantial advantage over the other in terms of providing ionic contributions to bonding.

Scheme 1 : Becker's synthesis of the lithium salt of PCO from 1992. [ 5 ]
Scheme 2 : Westerhausen's synthesis of the alkaline earth salts of PCO from 2002. [ 8 ]
Scheme 3 : Grutzmacher's synthesis of the sodium salt of PCO from 2011. [ 9 ]
Scheme 4 : Goicoechea's synthesis of the potassium stabilised salt of PCO from 2013. [ 10 ]
Figure 1 : The different resonance forms of the NCO and PCO anions. The values were calculated with B3LYP functional and aug-cc-pVTZ basis set using NBO/NRT analysis in GAMESS . [ 11 ]
Figure 2 : Reactions of the PCO anion which depict its ambidentate nature. [ 6 ]
Figure 3 : The HOMO and LUMO of the PCO anion; surfaces calculated with the B3LYP functional and aug-cc-pVTZ basis set using Molden and GAMESS . [ 11 ] From left to right, the atoms are P, C and O.
Figure 4 : The different reaction pathways of the PCO anion. [ 6 ]
Figure 5 : The different resonance forms and weights of the different ECX analogues. The values were calculated with B3LYP functional and aug-cc-pVTZ basis set using NBO/NRT analysis in GAMESS . [ 11 ]
Figure 6 : Stacked graph of Laplacian electron density of the PCO anion. Graph plotted in Multiwfn using the results of AIMS analysis from Molden and GAMESS . From left to right, the atoms are P, C and O.
Figure 7 : Contour plot of the Laplacian electron density of the PCO anion. Graph plotted in Multiwfn using the results of AIMS analysis from Molden and GAMESS . [ 7 ] From top to bottom, the atoms are O, C and P.
Figure 8 : Contour plot of the Laplacian electron density of the PCS anion. Graph plotted in Multiwfn using the results of AIMS analysis from Molden and GAMESS . [ 7 ] From top to bottom, the atoms are S, C and P.